The airline industry has a lot of issues, but it's not a monopoly. In fact, the reason we no longer have free bags is that the industry is in a state of price war, due to competition. Airlines want to reduce their ticket costs as much as possible, and this leads to little margin on the tickets, so they save costs by not giving free bags.

This helps stratify the market as well, because those who travel light (mainly business people and people who fly a lot) now pay less than they used to. Those who fly irregularly (i.e., tourists) are more willing to pay the fees.